Speed=distance /time ⇒ distance=Speed x time.
A=a city
B=other city.
320 miles/h 400 miles/h
----------------→ ←----------------
A-----------------------------------------------------------B
2160 miles
x=time
We can sugget this equation:
320x+400x=2160
720 x=2160
x=2160/720
x=3
Answer: 3 hours.